Allow system jobs to be exempted from app-standby

This flag will allow the sync manager to exempt certain kind of sync requests.

- This will only exempt jobs from app-standby.

- EBS still won't exempt them. Battery saver cuts background network anyway, so
background syncs will still not work, and it didn't used to work pre-P either.

- Manual force-app standby still doesn't allow them to run.

Bug: 72443754
Test: Build and boot. The behavior shouldn't change since none uses the flag yet.
Test: atest CtsJobSchedulerTestCases
Change-Id: I806b97bb4b7da773479e878e6eccb792b03eadc1